Techniques for forming Bragg gratings directly in the core of optical fibers have enabled a simple low cost manufacturing technology for optical filters, laser mirrors, wavelength division multiplexers and demultiplexers, chirped gratings for dispersion compensation, etc. The gratings in all of these devices have a very critical grating period .LAMBDA. which essentially determines the operating frequency of the system in which the grating is installed according to the relationship: EQU .lambda.=2.LAMBDA.n
where .lambda. is the effective resonant wavelength of the grating, .LAMBDA. is the period of the grating, i.e. the spacing between refractive index shifts, and n is the average refractive index. Both during manufacture, and in the service environment of the optical system in which the grating is used, environmental changes such as temperature affect both .LAMBDA. and n. The grating period, .LAMBDA., varies with mechanical strains on the optical fiber. Strains can be internal in the optical fiber itself, or can result from mechanical stresses produced in the fiber typically from differential thermal expansion of the elements in the package containing the optical fiber grating. Strains can also be introduced purposely, either during production of the fiber, or by design of the grating package. In the latter case, the strain can be a static strain, fixed by the structure of the package, or can be dynamic in the sense that the package is provided with means for adjusting the strain either as a manufacturing step, or in the use environment, to vary the effective resonant wavelength of the grating.
The ability to adjust strain in a fiber grating can be used to advantage either to change the resonant frequency of the grating or to preserve the resonant frequency of the grating by compensating against unwanted effects of temperature. Changing the resonant frequency is a useful expedient in the final stages of grating manufacture, or can be used as a device or system adjustment in, for example, WDM devices to add or subtract channels.
The relationship between strain in the fiber and the properties of the fiber is given by: EQU .DELTA..lambda.=.lambda.(1-P.sub.E).epsilon.
where .DELTA..lambda. is the change in the grating center wavelength, .lambda. is the signal wavelength, P.sub.E is the photoelastic constant (typically .about.0.22 for silica fibers), and .epsilon. is the strain in the fiber. Because the grating spacing is very small, these adjustments are typically very sensitive, with small strains in the fiber producing a significant shift in the resonant frequency of the grating.
Because of these and other considerations a variety of device packages have been designed to control strains in the optical fiber of fiber grating devices. Some of these are passive and depend on choice of packaging materials to minimize the effects of differential thermal expansion between the package and the fiber, or to compensate for temperature induced changes in the fiber itself. A typical prior art passive temperature compensating package is described by G. W. Yoffe et al, "Passive temperature-compensating package for optical fiber gratings", Applied Optics, Vol. 34, No. 30, Oct. 20, 1995, pp. 6859-6861. This paper also describes an active strain adjustment to set the strain of the fiber to the desired initial value. The active adjustment is realized by having one of the ends of the package fixed to the fiber but threaded to the rest of the package so that relative axial movement occurs between the fixed ends holding the fiber when the threaded portion is rotated. The degree of axial movement resulting from turning the threaded member is determined by the pitch of the thread. For a fine pitch thread of 80 threads per inch each turn of the threaded member produces an axial movement of 316 microns. For a package 40 mm in length, a single turn of the threaded adjustment member produces a strain in the fiber of close to 1.0%, well beyond the working strength of the fiber. Partial turns of the threaded member may provide useful adjustments in limited applications but this mechanism overall is undesirably coarse for most applications.
Other schemes for actively controlling the strain in an optical fiber grating have been proposed which provide better precision. For example, piezoelectric elements have been used to modulate the strain in the fiber in response to electrical signals. These devices are effective but costly. Similar packaging approaches using electrostrictive or magnetic strain control are also effective but are complex and prone to failure.